Method for manufacturing electric modules, and the electric module

ABSTRACT

A method is described for producing electric modules, where a power module is attached to a fastening part with an adhesive, with the adhesive first being precured in an edge area, and in another step the power module is encased in a gel. In another step, the gel and the adhesive are fully cured together in one step. This method is cost- and time-optimized for producing a compact electric module with power modules that have a high power loss and are exposed to high mechanical stresses.

BACKGROUND INFORMATION

Convention method and electric module are described in German PatentApplication No. 39 39 628, where the components are glued to ceramicfilm circuits with a UV curing adhesive or adhesive resin (abbreviated“adhesive”) and the adhesive is cured by irradiating the rear side ofthe ceramic film circuits with UV light. Curing of the adhesive beneaththe entire component is induced by the UV light fraction passing throughthe substrate.

SUMMARY OF THE INVENTION

A method according to the present invention has the advantage that afterprecuring of the independent method claim, however, has the advantagethat after precuring of the adhesive, the component or a power modulecan be encased in a gel in another step, and the gel and the adhesivecan be fully cured together in one step in a furnace operation after thegelation process. This means considerable time savings in comparisonwith two separate furnace steps for the adhesive and the gel. It can beregarded as an advantage of the electric module that it has a compactdesign which guarantees reliable contacting and cooling of thecomponents arranged on the power module.

Edge curing of the adhesive in an edge area of the power module, inparticular with UV irradiation, is also advantageous. Therefore, only asmall, freely accessible area is necessary for the curing radiation, andthe precuring process with UV light requires only a few seconds, so itleads to great time savings in comparison with curing in a furnaceprocess.

Precuring of the adhesive in conjunction with a power module bondingoperation has proven to be extremely advantageous. Precuring yieldsadequate initial bonding of the power module to a fastening part topermit fault-free contacting of the power module in an automated bondingoperation. In other words, precuring guarantees adequate fixation of thepower module for wire bonding as well as saving a great deal of time incuring the adhesive and the gel used for encasing the power module.

The power module is made ready in an advantageous manner by using ametal conductor frame made of copper in particular, applied to the chipwith an electrically conductive adhesive in an electrically conductingbond, for example. Here again, it is possible when curing the adhesiveto eliminate a furnace process, which would otherwise be time consumingand tedious in mass production, if the electrically conducting adhesiveescaping at the side of the chip is also cured with UV radiation or if,in order to produce an electric connection between the power module andthe metallic block, the adhesive is heated in an induction furnace tocure it, i.e., in a furnace where a suitable heating is achieved by eddycurrents induced by magnetic induction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of an electric module produced by a methodaccording to the present invention.

FIG. 2 shows a cross-sectional view of the electric module illustratedin FIG. 1.

FIG. 3 shows a design of a conductor frame used in the method accordingto the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a power module 6 arranged on a fastening part 1 designed asa heat sink. Power module 6 has a bottom piece designed as a copperblock 3 with one chip 4 and another chip 5 arranged on it. The two chipsare electrically interconnected by wire bonds 7. Heat sink 1 isconnected to a plastic casing 8, a detail of which is shown in FIG. 1. Anarrow edge of heat sink 1 can also be seen in recess 14 of electricmodule 15. The adhesive arranged between copper block 3 and heat sink 1is slightly visible beneath copper block 3, i.e., copper block 3 issurrounded in a narrow edge area by an edge area of adhesive layer 2.The chips may be connected electrically to the nickel-plated surface ofcopper block 3 by wire bonds 9, for example; likewise, in the embodimentillustrated, in FIG. 1 chip 5 is electrically connected to contact faces12 and 13 by wire bonds 10 and 11; these contact faces may be arrangedin the plastic casing and may lead, either inside the plastic casing oron its surface, to contact points of the electric module, e.g., to holeswhich also function to secure the electric module mechanically.

FIG. 2 shows a side view of a cross section of the electric module shownin FIG. 1. The same reference notation as used in FIG. 1 is used herefor the same parts and will not be described again. In the embodimentillustrated, the back of the heat sink is provided with cooling ribs,while on the front of the heat sink the power module arranged on theheat sink is surrounded with a gel 33 for the purpose of preventingcorrosion. This gel also surrounds contact faces 12, for example.Plastic casing 8 surrounds the arrangement of heat sink and powermodule, with heat sink 1 being joined in a form-fitting manner toplastic casing 8. The cross section here shows the electricallyconducting connection of chip 4 to copper block 3 by an additionaladhesive 32 which is electrically conductive. Electric module 15 is alsoprovided with a cover 30 which protects the area provided with a gelaround the power module from mechanical damage, for example.

Electric module 15 functions to charge an automotive battery, forexample, to a charge which depends on the charge state of the battery,by using a generator driven by an automotive engine. The module is thenexposed to harmful environmental influences and has a high power loss,so that both reliable mechanical protection and corrosion protection aswell as good dissipation of the power loss must be provided and yet thedesign must be as compact as possible. The heat sink and the adequatelyheat-conducting connection via copper block 3 and the adequatelyheat-conducting adhesive 2 guarantee reliable dissipation of the powerloss, the gel arranged around the power module provides protection fromcorrosion, and the cover and plastic casing 8 together with the contactpoints guarantee protection from mechanical damage and a sturdymechanical and electric connection of the module to other devices.

A method of producing an electric module illustrated in FIGS. 1 and 2 isdescribed below. First, a conductor frame 20 shown in to FIG. 3 isprovided, preferably being made of copper and having copper blocks 21which later form bottom part 3 of a power module 6. An electricallyconductive adhesive is applied with a stamp to copper blocks 21 ofconductor frame 20 to establish an electrically conducting connection(ground connection) between chip and copper block. In another step, thechips are arranged on the surfaces provided with the electricallyconductive adhesive and the adhesive is cured. This is done in a furnaceoperation and lasts for approx. 2½ hours. Solder which melts in thefurnace operation and thus establishes an electric connection betweenchip and copper block may optionally be introduced between the chips andthe copper blocks for mechanical fixation and electric contacting of thechips with the copper blocks. In an advantageous embodiment of thepresent invention, the presence of a metallic adhesive partner, namelythe copper blocks and the electrically conductive adhesive containingmetal particles, is utilized to achieve curing of the adhesive (ormelting of the solder arranged between the chips and the copper blocks)by magnetic induction in an induction furnace instead of a complicatedfurnace operation (time consuming and taking up space). As analternative, here again the chips may be secured by partial UV curing ofthe edge of a conductive adhesive which escapes somewhat at the sides.The prerequisite for this is the use of a UV-curing conductive adhesive.Complete curing of the UV-curing electrically conducting adhesive isaccomplished in another step together with the casting gel. The processsteps described above are part of preassembly of the power modules. Inaddition to whole chips, it is also possible to mount individualtransistors, diodes, triggering chips or regulator chips on a heat sink,in particular a metallic heat sink. The adhesive may be applied to theconductor frame by stamping, dispensing, screen printing or stencilprinting. The use of an induction furnace or a continuous UVpass-through zone leads to a definite shortening of the curing time ofthe electrically conductive adhesive and to a serial sequence of bondingwithout a buffer or stack, because the induction furnace can be designedsimply as a furnace with continuous flow through of power modules to becured. The cost of the above-described processes is lower in comparisonwith a traditional furnace operation with regard to energy andacquisition, because in particular there is no heat loss and only thoseelements (metallic elements) which are supposed to be heated are heated.Furthermore, the handling of the parts is simplified on the whole. Theinduction furnace has a continuous flow-through zone with magneticcoils. In addition to a shortened curing time, the warm-up phase of theconductor frame is greatly reduced by the use of magnetic radiation,because specifically metallic parts are heated. The conductor frameillustrated in FIG. 3 thus passes through the continuous flow-throughzone of an induction furnace or a UV irradiation installation, and thisprocedure permits integration of the aforementioned power modulepreassembly into the manufacturing process of the final assembly of theelectric module described below. This yields as potential savings lowerinvestment costs (one less furnace) and minimization of manufacturingtime per conductor frame plus low energy costs.

Final assembly comes after the preassembly of the power modulesdescribed above. The power modules are separated by punching outconductor frame 20, and wire bonding is optionally performed in a bondassembly, i.e., the various electric connections are established amongmultiple chips 4, 5 arranged on a bottom piece 3 designed as a copperblock 21. Power module 6 must then be attached to heat sink 1. Thisconnection is implemented by a heat conducting adhesive. Adhesive isapplied to the heat sink by stamping, dispensing or screen or stencilprinting. Then the power module is applied to adhesive layer 2. Inanother step, adhesive layer 2 formed by a UV-curing adhesive in an edgearea around power module 6 is precured, as shown in FIG. 1, where it islabeled with reference number 2. This edge crosslinking is performed byradiation curing with UV light at a wavelength of 10 to 400 nm,preferably at 300 to 400 nm, directed from above onto the power modulein the view according to FIG. 1. Only this edge area of the adhesivelayer of the UV-curing adhesive is crosslinked. This step takes approx.15 seconds. This precuring is sufficient to guarantee the mechanicalstability required for a subsequent wire bonding operation. In this wirebonding operation, the chips arranged on the copper block areelectrically connected to contact faces 12, 13 and/or to the copperblock by wire bonds 9, 10, 11 for example by ultrasonic welding. TheUV-curing adhesive used is, for example, a UV-curing acrylate adhesive.Such a UV-curing acrylate adhesive is available, for example, under thebrand name Vitralit 850/7 from the company Panacol. Spacers can be usedto advantage here to guarantee a minimum thickness of adhesive layer 2,which is necessary to compensate for mechanical stresses. In anotherstep, the power module and the wire bonds are cast in a gelling processwhere the power modules and the wire bonds are encased in a gel. Thenresidual curing of adhesive layer 2 beneath the power module (andoptionally adhesive layer 32) is performed, and the silicone gel usedfor the gelling process is cured in one operation in a furnace step. Inthe past, a heat-conducting adhesive which binds only through theinfluence of heat has been used for bonding a power module to a heatsink. Therefore, a furnace step had to be used between assembly of theheat sink with a power module and the wire bonding, because mechanicalsecuring of the power module is necessary for wire bonding. In themethod according to the present invention, however, instead of twoseparate furnace steps for adhesive curing and gel curing, each takingapprox. 1½ to 2 hours, only one common furnace step is necessary,whereas precuring of adhesive layer 2 is performed in a UV-precuringstep lasting 15-30 seconds, where the edge area of adhesive layer 2 isirradiated. In addition to the elimination of one furnace operation, itis possible to design the precuring operation as a continuousflow-through zone where modules with adhesive layers to be precured passthrough a zone filled with UV light continuously for a defined period oftime, starting from the preceding process step, then seamlessly enteringthe downstream process step of wire bonding. Since the IC is wirebonded, although the adhesive beneath the power module has not yet fullycured, the process of wire bonding here is also known as wet bonding.The advantages of the method of mechanical prefixation of the powermodule by initial edge curing of the UV-curing adhesive includeconsiderable cost savings, because it is not necessary to acquire asecond expensive furnace, and energy costs are reduced approximately toone half the energy required for full curing. Time savings of 1½ to 2½hours due to the fact that only one furnace step is required are alsoespecially advantageous.

The use of an induction furnace or UV edge curing of a conductiveadhesive as described in conjunction with preassembly of the powermodules is also conceivable when chips are bonded directly to a heatsink, which is surrounded by a plastic casing as shown in FIGS. 1 and 2.The advantage here is that only the metal is heated but the plastic isnot. Consequently, only the thermal mass of the metal but not that ofthe plastic need be heated. This also yields a definite shortening ofthe warm-up time and thus the total curing time. The prerequisite forthe use of an induction furnace is only that one of the adhesivepartners must have a high metal content so that heating can beaccomplished by induction currents in a controlled manner. Additionalapplications of an induction furnace could include the assembly of powertransistors and diodes in combined soldering and gluing operations,i.e., solder bonding or assembly of power modules on aluminum or ironheat sinks.

What is claimed is:
 1. A method for producing an electric module,comprising the steps of: a) preparing a power module having a metallicbottom piece; b) preparing a fastening part; c) applying an adhesive toa particular side of the fastening part; d) arranging the metallicbottom piece on the adhesive so that the adhesive forms an edge areaaround the power module; e) pre-curing the edge area by irradiating theedge area with a UV light, the UV light being directed onto the adhesivefrom the particular side of the fastening part; f) encasing the powermodule in a gel; and g) fully curing the gel and the adhesive together.2. The method according to claim 1, further comprising the step of: h)after step (e), wire bonding the power module.
 3. The method accordingto claim 2, wherein step (h) includes the substep of ultrasonic wirebonding the power module.
 4. The method according to claim 1, whereinstep (a) includes the substeps of: I. preparing a conductor frame andchips, II. introducing a bonding medium between the chips and theconductor frame, III. converting the bonding medium to at leastpartially bond the chips and the conductor frame, and IV. separating thepower modules.
 5. The method according to claim 4, wherein step (a)further includes the substep of: V. wire bonding the chips which arearranged on the power module.
 6. The method according to claim 4,wherein the bonding medium is at least one of a further adhesive and asolder.
 7. The method according to claim 11, further comprising the stepof: i) converting the bonding medium into an at least partially bondingstate by one of UV irradiating the bonding medium and heating thebonding medium in an induction furnace.
 8. The method according to claim1, wherein the fastening part includes a heat sink.
 9. The methodaccording to claim 8, further comprising the step of: j) joining theheat sink to a plastic housing.
 10. The method according to claim 8,further comprising the step of: k) placing a cover on the electricmodule.
 11. An electric module, comprising: a fastening part; a powermodule glued to the fastening part; and a casingjoined to the powermodule, wherein the casing is attached to the power module by bond wiresand contact faces, and wherein the power module is package with a gel.12. The electric module according to claim 11, wherein the casing iscomposed of a plastic material.
 13. The electric module according toclaim 11, wherein the power module includes one of a chip, a transistorand a further electronic component.
 14. The electric module according toclaim 11, wherein the power module includes an arrangement having chipson a bottom piece of the arrangement.
 15. The electric module accordingto claim 14, wherein the bottom piece is composed of a metallic block.16. The electric module according to claim 14, wherein the metallicblock is a copper block.